IsFET sensor for ph Measurement TopHit CPS 491

Transcription

1 Technical Information TI 377C/2/ae IsFET sensor for ph Measurement TopHit CPS 91 Ion-selective field effect transistor for long-term stability ph measurement in media with high solids content Application Areas Process applications with quickly changing ph values, alternating temperatures and pressures Water purificaiton and wastewater Media with high loads of: solids emulsions precipitation processes Used in hazardous areas; FM, CSA, and ATEX approved Features and Benefits Resistant to breakage no glass, sensor body made completely of PEEK direct installation into the process reduced effort and costs for sampling and laboratory analysis Double chamber reference system: poisoning resistant polyacrylamide free gel Application possible in particle containing media with high build-up potential Application possible at low temperatures short response time constant high accuracy Longer calibration intervals than glass electrodes Lower hysteresis with alternating temperatures Low measuring error after hightemperature loading Almost no acid and alkaline errors With built-in temperature sensor for effective temperature compensation The Power of Know How

2 Operating function and design Measuring principle Ion-selective (more general, ion-sensitive) field effect transmitters (IsFET) were developed in the 1970 s as an alternative to the glass electrode for ph measurement. Basics Ion-selective field effect transistors use a MOS (Metal Oxide Semiconductor) transistor arrangement (first figure below) where the metallic gate is not a control electrode. Instead, the medium in the IsFET is in direct contact with the gate isolator layer (second figure below). Two strongly N-conducting areas are diffused in the P-conducting substrate of the semiconductor material (Si). These N-conducting areas are current supplying ( Source ) and current accepting ( Drain ) electrodes. The metallic gate electrode (in case of the MOSFET), respectively the medium (in case of the IsFET), form together with the lower substrate to form a capacitor. A potential difference between gate and substrate (U gs ) causes a higher electron density between Source and Drain. A N-conducting channel is formed, feeding Source and Drain, a current (I d ) is induced. U gs 1 3 Si (n) S 2 Si (p) Si (n) D I d U d MOSFET 1 Metallic gate D Drain 2 Channel U d Drain voltage 3 p-doped silicon substrate U gs Control voltage between gate and source S Source I d Drain current U gs 6 5 Si (n) S Si (p) Si (n) D I d U d IsFET 1 Reference electrode S Source 2 Gate isolator layer D Drain 3 Medium U d Drain voltage Channel U gs Control voltage between gate and source 5 p-doped silicone substrate I d Drain current 6 Isolating layer (TopHit: sensor shaft) In the IsFET, H + ions available in the medium, which are located in the medium / gate isolator boundary layer, create the electric field (gate potential). Depending on the effect described above, a N-conducting channel is formed and a current between Source and Drain is induced. Suitable sensor circuits use the dependence on the ion-selective gate potential to create an output signal proportional to the concentration of the ion type. 2

3 ph sensitive IsFet Important characteristics of the TopHit CPS 91 The gate isolator serves as an ion-selective layer for H + ions. The gate isolator is impermeable to the ions as well (isolator effect), but allows reversible surface reactions with the H + ions. Depending on the acidic or alkaline character of the measurement solutions, functional groups in the isolator surface accept or reject H + ions (amphoteric character of the functional groups). This leads to a positive (H + acceptance in the acidic medium) or negative (H + rejection in the alkaline medium) charging of the isolator surface. Depending on the ph value, a defined surface charge can be used to control the field effect in the channel between Source and Drain. The processes, which lead to a creation of a charge potential and therefore to a control voltage U gs between Gate and Source, are described with the Nernst equation. U gs = U x RT Ig(a nf ion ) U gs = Control voltage between gate and source n = Mol weight U 0 = Offset potential F = Faraday constant R = Gas constant a ion = activity of ions T = Temperature (K) 2.3 x RT = Nernst factor nf At 68 F (20 C), the Nernst factor is known to have a value of -58 mv/ph. Resistance to breakage Its resistance to breakage is the most obvious feature of the sensor. The complete sensor is embedded in a PEEK shaft. Only the highly resistant isolator layer and the reference have direct contact with the medium. Acidic or alkaline errors A further important benefit compared to the glass electrode is the considerably reduced number of acidic or alkaline errors in extreme ph ranges. Compared to glass electrodes, practically no foreign ions can build up at the IsFET gate. The error detection limit is < 0.01 ph (between ph 1 and 13 at 77 F). The chart below shows the acidic or alkali error of the IsFET between ph 1 and 13 and the comparison to the glass electrode (two different ph glasses) at ph values of 0.09 and DpH E+H IsFET Glass 1 Glass M HCl ph 1 to 13 1 M NaOH ph 77 F (25 C) 3

4 Isothermic curves The Nernst equation (a) defines the dependence of the measuring voltage on the hydrogen ion content (ph value) and the temperature. It is the basis of ph measuring technology and for IsFET sensors as well. A temperature-dependent value for the potential change per ph value can be worked out from this equation (isothermic curve, potential changes per ph value at a defined temperature). The isothermic curves of the IsFET sensor are very close to the theoretical values. This is further proof for the high ph measurement precision of the TopHit H CPS 91. mv ph a b c Isothermic curves for TopHit CPS 91 a = Isothermic curve at 6 F (8 C): slope mv/ph b = Isothermic curve at 98 F (37 C): slope mv/ph c = Isothermic curve at 12 F (61 C): slope mv/ph Measurement stability and sensor response time The IsFET response times are very short over the whole temperature range. With the IsFET sensor, there is no equilibrium setting (temperature dependent) as in the source layer of a ph glass of a glass electrode. They can also be used to derive an application at low temperatures with a deceleration in response time. Large and fast temperature and ph value fluctuations have a significantly smaller effect on the measuring error (hysteresis) than with a glass electrode, as there is no stress exerted on the ph glass. Reference system The integrated reference electrode of the CPS 91 is a double-chamber reference system with a bridge electrolyte. The benefits are an efficient and stable contact between the diaphragm and the reference lead, and the extremely long poison path. The bridge electrolyte is highly resistant to temperature and pressure changes. Measuring system The complete measuring system consists of: IsFET sensor CPS 91 Measuring cable CPK 12 with TOP 68 connector Measuring transmitter; Liquisys M CPM 223 (for panel mounting), CPM 253 (for field mounting), or Mycom S CPM 153 Immersion, flow or retractable holder assembly, e.g. CleanFit P CPA 71 or 75 Additional accessories are available depending on the application: TopClean S CPC 30 or TopCal S CPC 300 automatic cleaning system Extension cable, VBA or VBM junction box.

5 Food industries zone 0/1 Hazardous area zone 1 Non-hazardous area CPS 91 with TopCal fully automated measuring, cleaning, and calibration system 1 CPS 91 sensor 5 CPG 300 control unit 9 Rinse block 2 Retractable holder CPA 75 6 Steam, water, cleaner 10 Electric cable 3 Mycom S CPM 153 analyzer 7 Cleaner, buffer solutions 11 Compressed air Power supply 8 Multihose 12 Liquids/cleaner Process sterilizing is no problem due to the wide range of applications for the IsFET ph sensor, not only relating to temperature but also ph. There is only a small range of extremely high ph values at high temperatures where the sensor is not suitable (see Process conditions). Media with these characteristics remove the isolator oxide from the IsFET chip. As this is the ph and temperature range of CIP cleaning media, the CPS 91 should only be used in combination with an automatic retractable holder. Benefits of the TopCal fully automatic measurement, cleaning and calibration system: CIP cleaing is possible - the sensor built into the retractable holder is automaically retracted out of the medium before cleaning Sensor cleaning in the rinse chamber of the retractable holder with suitable cleaning solutions Calibration cycles can be individually set Low maintenance costs due to fully automatic cleaning and calibration functions Measuring results are optimally reproducable and the individual value tolerances are very low due to automatic calibration 5

6 MEAS DIAG CAL PARAM TopHit CPS 91 Water and wastewater CPS 91 electrode 2 CPA 111 Immersion holder 3 CPK 12 measuring cable Liquisys M CPM 253 measuring transmitter Input Measured values Measuring ranges ph value and temperature 0 to 1 ph, 5 to 275 F (-15 to 135 C) Caution! Note the process operating conditions and notes on page 9. Performance characteristics Reference operating conditions Response time Maximum measured error Repeatability Start-up drift Reference temperature, 77 F (25 C) Reference pressure, 1.7 psia (1013 mbar) < 5 seconds for buffer changes from to 7 ph under reference operating conditions ± 0.2% of measuring range; temperature, Class B according to DIN / IEC 751 ± 0.1% of measuring range Everytime when switching on the measuring device, a control loop is set up. During the this time, the measured value moves to the true value. The setting time depends on the kind of interruption and the interruption time: Supply voltage interruption, senosr left in process: approximately 3 to 5 minutes Interruption of the fluid film between ph sensitive IsFet and reference lead: approximately 5 to 8 minutes Longer dry storage of the sensor: up to 30 minutes. 6

7 Electrical connection Measuring cable The IsFET TopHit H CPS 91 is connected to the measuring transmitter using the CPK 12 measuring cable. NOTE: Refer to the transmitter operating instructions for sensor wiring. The transmitter specified must be appropriate for the use of IsFET sensors, e.g. Liquisys S CPM 223 / 253. Transmitters with standard ph input only will not work with the IsFET electrode. Cable colors yellow and white are connected on the sensor side Source SRC (ph) Reference Drain DRN Pt1000 Pt1000 Pt1000 PAL GND Coax cable Shield RD GY WH YE BN Shield 7

8 Installation Sensor orientation In principle, IsFET sensors can be installed in any position, as there is no liquid inner lead. When installing the TopHit H CPS 91, note the flow direction of the medium. The IsFET chip should be located at a 5 to the direction of flow (right illustration). The rotating plug-in head of the CPS 91 allows easy installation at a defined angle. NOTE: Comply with the appropriate operation / installation guidelines for the electrode holder specified. 1 2 z x 3 x 5 z z y 6 1 engraving 2 Serial number 3 Nameplate y 1 engraving Rotatable part of connection head 5 IsFET chip 6 Medium flow-past direction 5 When installing the sensor in a holder, use the engraved serial number on the connection head for correct sensor orientation. The serial number is always located in the same plane as the IsFET chip and the nameplate (z-y direction). In addition, you can use the logo engraving for correct sensor orientation. The log is rotated by 70 to 90 form the IsFET plane (position 1, right hand graphic above). Installation angle IsFET sensors can be installed in any position, as there is no liquid inner seal. In the case of an overhead installation, a possible air cushion * in the reference system might interrupt the electrical contact between the medium and the diaphragm. * The sensor is shipped without air cushions. Air cushion forming is possible when tanks are cleaned out. NOTE: The installed sensor may be held under dry conditions for a maximum of 6 hours (also applies to overhead installaiton). advised angle of installation Permissable with respect to the application specific conditions 8

9 Environment Ambient temperature range Stroage temperature range Ingress protection Sensitivity to light 5 to 28 F (-15 to 10 C) The ambient temperature should not drop below 5 F (-15 C). Do not operate the sensor at temperatures below 5 F (-15 C) due to danger of frost damage. 32 to 122 F (0 to 50 C) NEMA 6 (IP 68) with TOP 68 plug connector As with every semiconductor, the IsFET is light-sensitive (fluctuations of measured value). Avoid direct sunlight during calibration and operation. Normal environment light does not influence the measurement. Process Medium temperature depending on ph At high temperatures over a long period of time, alkalis irreversibly destroy the gae isolator oxide. The sensor can only be used in the indicated range (chart below) at a cost of its life span. If it is constantly subjected to the effects of a 2% sodium hydroxide solution at 176 F (80 C), the sensor life span drops to approximately 10 to 15 hours. F ( C) 28 (120) 212 (100) Restricted lifetime! 176 (80) 10 (60) 10 (0) application range of IsFET TopHit 68 (20) 32 (0) Temperature and ph ph Application at low temperatures Pressure-temperature diagram Application range of the sensor according to the order code (see Ordering information) Pressure / temperature: 15 psi / max. 212 F (10 bar / 100 C), sterilizable: 3.5 psi / 230 F (3 bar / 110 C) for up to 1 hour Caution! To prevent damage to the sensor, never use the CPS 91 in applications outside the given specifications F ( C) 230 (110) 212 (100) Application range of TopHit 3.5 (3) 15 (10) psi (bar) 9

10 Mechanical construction Dimensions 1.72", 8.86, 1.2, 16.7 (120, 225, 360, 25) " (12) 1 Top 68 plug-in head 2 Sensor shaft 3 Reference electrode IsFET chip 5 Seal (perflurorelastomer) Weight Materials Surface roughness Temperature sensor Process connection Plug-in head Hazardous area approvals 3.5 ounces (0.1 kg) Electrode shaft: PEEK, conforms to FDA requirements Perfluoroelastomer seals; diaphragm is ceramics R a < 0.8 µm (150 grit) Pt 1000 (Class B to DIN IEC 751) Pg 13.5 ESB: TOP 68, rotatable Certificates and approvals FM approved IS, Cl. I, Div. 1, Grps. A-D when used with Mycom S 153-O / -P CSA approved IS, Cl. I, Div. 1, Grps. A-D when used with Mycom S 153-S ATEX device group II, Category 1 G; explosion protection EEx ia IIC T/T6 Accessories Transmitters Fully automatic measuring systems Liquisys M CPM 223 / 253: ph and ORP transmitter, field or panel mounted housing, for use in hazardous or nonhazardous areas, HART and PROFIBUS available. Mycom S CPM 153: ph and ORP transmitter, one or two channel version, for use in hazardous or nonhazardous areas, HART and PROFIBUS available. TopCal S CPC 300: fully automatic measuring, cleaning and calibration system, for use in hazardous or nonhazardous areas, in-process cleaning, automatic sensor monitoring. TopClean S CPC 30: fully automatic measuring and cleaning system, for use in hazardous or nonhazardous areas, in-process cleaing, automatic sensor monitoring 10

11 Sensor holder assemblies CleanFit P CPA 71: retractable assembly for tank and pipe installation CleanFit P CPA 73: pneumatically or manually driven retractable assembly with ball valve for reliable process termination, wetted parts are 316L SS CleanFit P CPA 7: pneumatically or manually driven retractable assembly with ball valve for reliable process termination, wetted parts are PP, PEEK or PVDF CleanFit P CPA 71 CleanFit P CPA 73 CleanFit P CPA 7 CleanFit H CPA 75: retractable assembly for tank and pipe installation under sterile conditions CleanFit W CPA 111: immersion and installation assembly for open and closed tanks UniFit H CPA 2: installation assembly for food industry, biotechnology and pharmaceutical industry, with 3-A or EHEDG certificate CleanFit H CPA 75 DipFit W CPA 111 UniFit H CPA 2 ph buffer solutions Buffer solutions, accuracy 0.02 ph, according to NIST / DIN ph.0 red, contents: 3 oz (100 ml) Order No.: CPY 2-0 ph.0 red, contents: 1qt (1000 ml) Order No.: CPY 2-1 ph 7.0 green, contents: 3 oz (100 ml) Order No.: CPY 2-2 ph 7.0 green, contents: 1qt (1000 ml) Order No.: CPY 2-3 Buffer solutions for single use, accuracy 0.02 ph, according to NIST / DIN ph.0 red, contents: 0.6 oz (20 x 20 ml) Order No.: CPY 2-D ph 7.0 green, contents: 0.6 oz (20 x 20 ml) Order No.: CPY 2-E Supplemental documentation Holder assemblies Transmitters Fully automatic systems Measuring cable Cleanfit P CPA 71 Technical Information, TI 217C/2/ae Cleanfit P CPA 73 Technical Information, TI 3C/2/ae Cleanfit P CPA 7 Technical Information, TI 35C/2/ae Cleanfit H CPA 75 Technical Information, TI 20C/2/ae DipFit W CPA 111 Technical Information, TI 112C/2/ae DipFit P CPA 10 Technical Information, TI 178C/2/ae CPA 2 Technical Information, TI 297C/2/ae Liquisys S CPM 223/253 Technical Information, TI 19C/2/ae Mycom 153 Technical Information, TI 233C/2/ae TopClean Technical Information, TI 235C/2/ae TopCal Technical Information, TI 236C/2/ae Measuring cable CPK 1-12 Technical Information, TI 12C/07/en 11

12 Ordering information TopHit H CPS 91 sensor CPS Shaft length 2.7 (120 mm) shaft length 8.9 (225 mm) shaft length (360 mm) shaft length (25 mm) shaft length 2 Terminal head ESB Threaded plug-in head, Pg 13.5, TOP 68 connection, rotatable 3 Options 2 Chip seal: Perfluoroelastomer 9 Special version according to customer specification CPK 12 measuring cable CPK Cable length HA 16 ft (5 m), TPE sheath, max. 266 F (130 C) HB 33 ft (10 m), TPE sheath, max. 266 F (130 C) HC 50 ft (15 m), TPE sheath, max. 266 F (130 C) HD 65 ft (20 m), TPE sheath, max. 266 F (130 C) HE 82 ft (25 m), TPE sheath, max. 266 F (130 C) HF 5 to 20 m (sold in m lengths), TPE sheath, max. 266 F (130 C) HG 16 to 65 ft (sold in ft lengths), TPE sheath, max. 266 F (130 C) HH 10 ft (3 m), TPE sheath, max. 266 F (130 C) NA 16 ft (5 m) PVC sheath, max. 176 F (80 C) 2 Version A Standard applications, black sheath Z Hazardous applications, Blue sheath 3 Wire termination 1 End sleeve on device side, braided cable shielding Ground connection A External potential matching with flat plug For application and selection assistance, in the U.S. call 888-ENDRESS For total support of your installed base, 2 hours a day, in the U.S. call Visit us on our web site, United States, Inc Endress Place Greenwood, IN 613 Phone: (317) ENDRESS FAX: (317) Canada Canada Ltd. 10 Graham s Lane Unit 1, Burlington ON, L7S 1W3 Phone: (905) FAX: (905) Mexico Paseo del Pedregal No. 610 Col. Jardines del Pedregal 01900, Mexico D.F. Mexico Phone: (525) FAX: (525) The Power of Know How TI 377C/2/ae/ , Inc.